The invention relates to seal clearances in rotary machines and, more particularly, to a static seal for a turbine assembly providing for greater clearance during transient operation and tighter clearance during steady state operation.
Rotary machines include, but are not limited to, gas turbines and steam turbines. The moving part of the turbine is called a rotor, and the fixed, non-moving part, i.e., housings, casings etc. is called a stator. Usually, the rotor rotates within a stator assembly at very high speeds, powering a generator, which in turn produces electricity or power.
A steam turbine has a steam path that typically includes, in serial-flow relationship, a steam inlet, a turbine, and a steam outlet. A gas turbine has a gas path, which typically includes, in serial-flow relationship, an air intake (or inlet), a compressor, a combustor, a turbine, and a gas outlet (or exhaust nozzle). Gas or steam leakage, either out of the gas or steam path or into the gas or steam path, from an area of higher pressure to an area of lower pressure, is generally undesirable. For example, gas path leakage in the turbine or compressor area of a gas turbine, between the rotor of the turbine or compressor and the circumferentially surrounding turbine or compressor casing, will lower the efficiency of the gas turbine leading to increased fuel costs.
Tight radial clearances are important to achieving high efficiency. Turbine operation at off-design conditions often means that the rotor and stator interfere, causing the turbine to “rub.” Clearances can be increased to avoid rubs, but with a loss of turbine performance.
Abradable coatings have been developed for use on stator seals. The presence of these coatings allows the rotor to interfere with the stator without permanent damage to the rotor seal teeth. Instead, the rotor rubs away part of the coating on the stator seal. Other turbines use abradable material, such as honeycomb metal, to achieve the same result.
Typically, when a turbine is shut down after some period of operation, a turning gear is used to keep the rotor turning slowly to prevent uneven cooling. On rare occasions, the rotor seal teeth will penetrate the stator seal coating (abradable coating) during or after the turbine shutdown. This can be due to the nature of turbine operation, thermal or other distortion of the turbine rotor and/or stator, or dimensional variation in the turbine components or any combination of these. If the penetration is deep enough and affects multiple seal teeth, friction between the rotor and stator can overwhelm the turning gear capability, and the rotor can become “locked up.”
As metal temperatures approach ambient air temperature, the turbine will return to its as-designed cold shape, and the rotor will free itself from the stator. Unfortunately, this process can take several days. An outage of several days is unacceptable to the turbine operator due to the loss of revenue.
It would be desirable to modify the stator seal such that an extended outage can be avoided.